The epicardial leads may be prescribed for the stimulation of the left ventricle, as an alternative to pacing leads implanted via the coronary sinus, which require a delicate approach for their implantation and are not without various drawbacks.
Nevertheless, unlike endocardial leads introduced via the venous network (see for example U.S. Pat. Publication No. 2009/0157136 A1), the implantation of an epicardial lead constitutes a very invasive operation, usually requiring general anaesthesia and the use of surgical techniques. It is indeed necessary that a chest surgeon incises the thorax so as to provide access to the pericardial sac (the pericardium being the fibro-serous envelope that surrounds the heart) and the myocardium itself, in order to fix the lead on the external wall of the latter, by suturing or by screwing.
For this reason, the implantation of an epicardial lead, though representing a known technique, is often seen as a back-up solution in case of failure of implantation of a lead through the coronary sinus. Moreover, an epicardial lead often provides poor results, especially with respect to electrical performance.
Several types of epicardial leads have been proposed, but all have serious enough drawbacks. A first type of lead uses an electrode pressed against the wall of the epicardium, wherein it is supported by suturing. These leads are very stable, but require broad access to allow the suture by the surgeon, and the possible implantation zone is very limited, as it is restricted to the vicinity of the chest incision.
A variant of this type of lead is a lead body that is divided into two distinct branches, each branch having a distal end supporting a pacing electrode to be sutured on the cardiac wall. This support is provided by an absorbable suture terminated with a needle: during the procedure, after suturing the electrode to its support on the cardiac wall, the surgeon buries the absorbable suture using the needle, which needle is subsequently removed by cutting the suture. This permanent traction effort provides excellent contact of the electrode, positively biased against the heart wall. The presence of two separate electrodes also allows application of a bipolar stimulation, over a somewhat wider surface area.
Another type of lead proposed is equipped with an anchoring helical screw for screwing in the myocardium wall. Screwing can be performed directly, but the work area is then limited in the same way as a sutured lead. It can also be performed using a special insertion tool having an articulated head on which the lead is mounted, but the area of possible implantation, although broader, is nevertheless limited by the rigidity and the large diameter of the support tube of the instrument that the surgeon must manipulate in the curved pericardial space.
Moreover, from the mechanical point of view, the screws of current leads are very generously sized, due to the mechanical stresses exerted on the screw during implantation, arising from the high amplitudes of displacements and the radial tractions that are exerted. These generously sized screws are relatively traumatic to the tissues, with possible creation of local fibrosis reactions.
One disadvantage which is common to all these leads, besides the highly invasive surgery, is their relatively poor electrical performances, particularly because of the large size of the sutured electrode (which should ensure a satisfactory contact with the heart wall) or because of the large dimensions of the screw (needed to withstand implantation constraints). But a large electrode does not provide a satisfactory current density, which is detrimental to the stimulation efficiency.
In addition, the stimulation is punctual (only one stimulation site), with two drawbacks:                A less effective stimulation compared to a multisite configuration, and        The risk that the chosen site is not the most effective, or that because of cardiac remodeling, the site originally chosen over time become less effective. In this regard, given the invasive nature of the operation, it is not a realistic option to consider further surgery on the patient to try to improve the situation by seeking a possible alternative site that would be more effective than the originally chosen site.        
A final drawback of existing epicardial leads, particularly for screw leads, is that it is almost impossible to extract them once they are implanted.
For all these reasons, the use of epicardial leads remains undeveloped, and these leads are generally used as a last resort when other techniques are not feasible.
The U.S. Pat. Publication No. 2007/0043412 A1 proposes a device implementing multiple electrodes (of a conventional type) located in different parts of the epicardium to form a network of electrodes. From an electrical point of view, each electrode is provided with its own connecting conductor, and the respective conductors are connected together to a common conductor connected at its other (proximal) end to the generator. The pacing pulses can thus be simultaneously spread to several electrodes at several points of the myocardium, corresponding to the points of implantation of the electrodes.
WO 2005/039690 A1 describes a different lead configuration, wherein two screw epicardial electrodes are implanted facing each of the two ventricles, each electrode being connected to the generator by its own conductor.
With these devices, since the electrodes used are of a conventional type (typically, screw electrodes), the problems mentioned above related to the nature of the electrodes remain, including: difficulty of implantation with the use of complex instruments particularly for in situ screwing after the implantation position is reached; need of a highly invasive intervention, which is traumatic for tissues with possible creation of a local fibrosis reaction of large sized screws because of mechanical constraints; poor electrical performances because of the large size of the electrode, which does not provide a satisfactory current density, to the detriment of the stimulation effectiveness.